Conductive inks are used to form these conductive grids or metal contacts. Conductive inks typically include a glass frit, a conductive species, such as silver particles, and an organic medium. To form the metal contacts, conductive inks are printed onto the substrate in a pattern of grid lines or other pattern by screen printing or other process. The substrate is then fired, during which electrical contact is made between the grid lines and the substrate. This contact is enhanced by the formation of individual silver crystallites at the glass-substrate interface. Without being bound by theory, it is believed that charge carriers are transferred from the substrate to the silver crystallites and then transferred to the gridline either through the glass layer by tunneling or directly to the silver of the gridline, if there is direct contact of the crystallite with both the gridline and the semiconductor. Lower firing temperatures are desirable in this process because of the lower cost involved and energy saved.
As otherwise mentioned herein, the anti-reflective coating enhances light absorption but also acts as an insulator which impairs the excited electrons from flowing from the substrate to the metal contacts. Accordingly, the conductive ink should penetrate the anti-reflective coating to form metal contacts having ohmic contact with the substrate. To accomplish this, conductive inks incorporate glass frits to aid with sintering silver particles to a substrate and to promote adhesion and ohmic contact between the formed metal contact and the substrate. When the glass frit liquefies, it tends to flow toward the interface between the silver particles and the anti-reflective coating on the substrate. The melted glass dissolves the anti-reflective coating materials as well as some of the metal particles and substrate. Once the temperature decreases, the molten silver and the melted or dissolved substrate recrystallize through the liquid phase. As a result, some of the silver crystallites are able to penetrate the antireflective layer and form ohmic contact with the substrate. This process is referred to as “fire-through” and facilitates a low contact resistance formation and a stronger bond between conductive grid or metal contact and the substrate.
The automotive, electronics and solar cell industries place greater emphasis on using environmentally-friendly components and processes. This emphasis has been further urged by the need to comply with environmental regulations. In response, the solar cell industry is moving to eliminate the use of lead in components and materials used in solar panels of cells.
Accordingly, there is a need for a lead-free glass frit which can be fired at a lower temperature and that can penetrate the anti-reflective layer and form metal contacts in ohmic contact with a substrate.